Typecast dependency injected classes to their interface

Problem/Motivation

When using dependency injection, the calling code can provide a different implementation of a dependency as long as it follows the same interface. This is a common practice is testing scenarios and profiling, for example Drupal uses this in Simpletest and in the Webprofiler contrib module.

When the dependency is typecast to a specific class, the service is no longer swappable. Depending on the calling code, this can result in notices or errors.

Proposed resolution

Replace typecasted dependencies with their associated Interface when available.

Remaining tasks

1. Write a patch
2. Review
3. Commit

User interface changes

None.

API changes

None.

Data model changes

None.